(1) Field of the Art
This invention relates to the so-called hot isostatic pressing (hereinafter referred to simply as "HIP" for brevity) for sintering, consolidation or diffusing bonding of metal or ceramic powder compacts in a high pressure and temperature gas atmosphere.
(2) Description of the Prior Art
The HIP treatment which isostatically compresses an item to be worked on using an inert gas as a pressure medium at high temperatures has been attracting the special attention of many concerns as an excellent technology which can be applied for the purpose of removing residual pores in cemented carbides by crushing, for producing sinters of high density from metal powder or ceramic powder or mixtures thereof, or for diffusing bonding of metallic materials. With regard to the HIP treatment, various proposals have been made, including a method of using a secondary pressure medium (Japanese Laid-Open Patent Application No. 47-30508), a method of using a metal capsule of a complicated shape, a method of using a glass capsule (Japanese Patent Publication No. 46-2731), a method of embedding a preliminarily molded work in glass powder and compressing the same by a high pressure gas after melting the glass (Japanese Laid-Open Patent Application No. 55-89405), etc. Of these proposals, the method of the aforementioned Japanese Laid-Open Patent Application No. 55-89405 is superior to other methods in that it requires neither a secondary pressure medium nor a special capsule or a gas-tight casing for sealing in a work.
However, in the just-mentioned method, it is required in the first place to heat the HIP system to melt the glass powder in which the preliminarily molded item to be worked on is embedded. However, as is well known in the art, the time required for raising the temperature to a level where the glass particles soften and melt varies largely under a pressurized condition and under atmospheric or reduced pressure, taking a considerably long time under atmospheric pressure. For example, high pressure argon gas of 1000 kg/cm.sup.2 which is several hundreds times greater than argon at the atmospheric pressure in density is as low as 1.1 to 3 in viscosity, so that there occur vigorous convections, increasing the rate of convection heat transfer to an extremely great value. Accordingly, the rate of heat transfer from the furnace atmosphere to the work is escalated a considerable degree. If the furnace atmosphere gas is maintained at high pressure in heating, however, there arises a problem in that the high pressure gas is occluded in the melted glass and the preliminarily molded work, forming pores or defective structures in the final product to be obtained after the HIP treatment. It follows that the glass powder has to be melted by heating in a vacuum or in an inert gas atmosphere at most at the atmospheric pressure. Therefore, there will be required a longer time period for scavenging and gas replacing operations in addition to the longer heating time due to the drop in the heat transfer rate, lowering the operational efficiency of the relatively costly HIP system and as a result inviting increase in the production cost. To cope with the recent demand for larger products, there is a trend toward larger HIP systems, which involve a longer time period for each HIP cycle. In order to overcome this problem, there have been adopted various means for enhancing the efficiency of the HIP system, including the provision of a compressor of higher efficiency and capacity and improvement of the heater. The improvement of the HIP system itself has limitations and incurs large expenses.